C100 Channel/speaker issue *solved*

[Dbzerk]

Hi all.
I'm feeling like it could be a bad amp chip but I'm hoping someone could shed some light on my issues with the sound on a C100 that I'm restoring.
The main issue is that both speakers only work on the 'wide' setting, stereo and mono the right speaker works.
This is what I've observed.

Playing cassette or AUX in:
Stereo and Mono= only right speaker
Wide= both speakers work fine except its louder when balanced to right channel.

Radio mode:
Mono= only right channel plays fine
Stereo & Wide= No out pur from speakers.. faint sound only from right channel

Headphones:
Only balanced to right does it work, (on both earphones (might be dodgy earphones)) No sound when balanced to left.
Wide, stereo, mono has no effect on this.

I'm thinking it could be the amp, although I am getting output from both speakers in wide, so I'm a little confused.

Thanks in advanced

 

[baddboybill]

Sounds to be some sort of right channel issue. I would clean all switches and pots thoroughly and even headphone jack. Start of easy. The wide switch I believe uses both channels so in theory the left channel is coming through right and vise versa

 

[Dbzerk]

Hey mate. I've had the whole thing apart, opened and cleaned every pot and switch..it was a problem I had with the box before i began restoring it, i was hoping it would clear up during the pot/switch cleaning/deoxit..
If sound is coming through both speakers in wide setting wouldn't that suggest that the 2 Amps are working fine and the issue could be with the pre-amps? I can't find any info about how the wide setting works

 

[floyd]

I know it sounds stupid but sometimes cleaning the headphone jack helps.

 

[Dbzerk]

Humm.. I've already deoxit it, but I'll look at it a bit more thoroughly or even bypass the switch in it ..

 

[Transistorized]

Have you tried cleaning the record bar on the lower deck? This can intercept the sound before it reaches the main amp. Sometimes you have to hose it down to where its dripping off the board and work that record bar back and forth many times. (Power unit off and remove power source when doing this). Clean excess off board and allow to dry a few minutes before testing again.

Prior to cleaning, I would pop in a tape, press record to work that switch on the board and record off the aux input. Watch the meters. Do they both swing equally for left and right channel? If so the preamp is seeing the signal. Does the tape when played back sound normal? Play it back in another cassette deck and see if the issue is recorded onto the tape.

Sometimes a dirty record bar can be cleared up just by engaging the record feature on the deck

If record bar doesn't resolve, I would focus my attention to the Function selector switch (since you are getting different results on different input selections). Follow the blue selector ribbon to the function slider switch. Have you cleaned it? Also clean the stereo wide mono switch if not already.

I do not believe you have a bad chip. Audio output chips rarely fail unless something or someone has shorted the outputs at some point.

Speaking of output. You may also try cleaning the speaker output Jack's on the rear. They rarely get used or thought about. A bad connection there could intercept the output stage to the onboard speaker. While you're back there, also deoxit the phono/aux selector switch. This can cause Aux input issues.

Try all of this and get back with us.

 

[Dbzerk]

Thanks for the reply mate. I've already had desoldered the record bars and all the other switches and pots for a proper deep clean.. no change from how it was prior to rebuild, except static was gone.


Next year I'll try out the other stuff you've suggested.. hopefully it's a simple fix.

For now I'm off to Mauritius to get married

Merry Christmas

 

[Dbzerk]

Have you tried cleaning the record bar on the lower deck? This can intercept the sound before it reaches the main amp. Sometimes you have to hose it down to where its dripping off the board and work that record bar back and forth many times. (Power unit off and remove power source when doing this). Clean excess off board and allow to dry a few minutes before testing again.

Prior to cleaning, I would pop in a tape, press record to work that switch on the board and record off the aux input. Watch the meters. Do they both swing equally for left and right channel? If so the preamp is seeing the signal. Does the tape when played back sound normal? Play it back in another cassette deck and see if the issue is recorded onto the tape.

Sometimes a dirty record bar can be cleared up just by engaging the record feature on the deck

If record bar doesn't resolve, I would focus my attention to the Function selector switch (since you are getting different results on different input selections). Follow the blue selector ribbon to the function slider switch. Have you cleaned it? Also clean the stereo wide mono switch if not already.

I do not believe you have a bad chip. Audio output chips rarely fail unless something or someone has shorted the outputs at some point.

Speaking of output. You may also try cleaning the speaker output Jack's on the rear. They rarely get used or thought about. A bad connection there could intercept the output stage to the onboard speaker. While you're back there, also deoxit the phono/aux selector switch. This can cause Aux input issues.

Try all of this and get back with us.

Well I'm back at home and back to the conion.
Thanks so much for the great tips Transistorized.

I've gone through and desoldered and pulled apart/cleaned ever selector switch and potentiometer.
Still no go on the left channel in stereo or mono mode. This fault is common to every function setting, tape 1&2, aux and radio.

I went and did what you suggested also and recorded to tape from auxiliary and also radio. Theres nothing being recorded to the left channel. Would this suggest a preamp issue?

 

[Superduper]

Speaker jack, headphone jack, etc are all after the output amplifier. IF you can get the left speaker to work in the wide or mono mode, THE AMPLIFIER IS MOST CERTAINLY OK. Think about this for a second, why in the world would cleaning the headphone jack clear up a no L audio condition in stereo mode, when you can get audio in mono or wide? The headphone jack doesn't care... as long as it's getting sound from the amp, it will play it regardless of wide/mono/stereo.

So is the problem in the preamp? Maybe. Probably. But a more accurate observation would be that the problem is upstream of the output amplifier. That's because boomboxes don't have a typical preamp like in a separates type of system. The preamp is virtually any circuit before the output amplifier. It could be anywhere. A broken connection, a poor or fractured solder joint. An open coupling capacitor or open resistor in the signal path. One of the preamp chips, or preamp buffer amps (transistors). Whenever I consider a problem, I think about how the signal travels during the various modes:

Stereo: 2 parallel and discrete signal paths. One for R and one for L. They terminate at the speakers. If there is a break anywhere in one channel, that channel will present as dead.

Mono: The 2 discrete signals are mixed together. Therefore, even if only one channel was sent into the line-in jacks, (try it-- dangle one RCA), you'll notice that both speakers will normally play. However, this is only the case if the signal break happens BEFORE the mono/stereo switch. If the signal break is after the switch, the dead channel might still be dead. Normally mono is a "mix" of the L and R channels, but if one channel has a break in the signal path, the sound from the speakers, even though both are playing, won't be a "mix" of the L and R but will only be the remaining live channel playing on both speakers.

Wide: The 2 discrete L/R channels are sent to the amp normally, but (in most designs), the L and R signals from the outputs (after output amplifier) are attenuated and reintroduced back into the pre-amplifier stage with a delay, criss-crossing into the opposite channel. This is why a dead channel, even if all the way back to the beginning, will seemingly be restored if in the "wide" mode. It won't sound like a normal expanded soundstage, but only the remaining live channel with a slight delay in the remaining seemingly restored channel.

The symptoms you reported is classic symptoms of a dead channel somewhere. The best way to get this sorted out is to use either signal tracing or signal injection while following the circuit diagrams. If that's beyond your ability, then you can try to redo all the solder joints and clean all connections and check all wires from end to end to make sure there are no breaks. Hopefully, that's where the problem is. But if the problem is an open resistor or cap, then you'll have to check them all to see if any are dead and this may require you to lift one leg on each component to test, which is very tedious. Signal tracing or injection are active tests whereas resistor or cap testing with DMM are passive tests. You might even be able to test the transistors with a passive test, but if a preamp IC is bad, that's difficult, maybe impossible to do with a simple VOM.

 

[Dbzerk]

Thank you so much for the valuable information, Superduper.
I guess i was hoping it was a fault that was something that came up frequently on the c100..😥.
I have found Cautions block diagram and a circuit diagram on here so I'll use your advice and go with tracing the signal.
Everything is still pulled apart, so hopefully it won't take too long to find it.

Thanks again everyone and to this site.
I'll let you know how I go with it.

 

[Dbzerk]

Finally had time to sit down and fix this problem.
I built a signal tracer as Super recommended, found out there was a broken track leaving the mode selector.





Bypassed it with a bit of wire, all is PERFECT with the beast.

Also had a bit of time to trouble shoot the other c100..
It was making a very loud feed back humming noise then went into a loud 'oscillating' sound..
I used one of these transistor testers and found out to have a faulty Q204 transistor. Swapped it with the know good one from the other board to confirm the fault.



Now to find a replacement D325

Thanks to all for the great help

 

[caution]

Cool! I bet you feel pretty good right now
Q204's collector ties directly to main power, so that was a good transistor to check. Great work getting it sorted.

 

[Dbzerk]

Yup I am pretty over the moon at the moment. Hahah

 

[docs]

Boom, glad you fixed her.

 

[Transistorized]

When I see this stuff...it's like watching a movie with a happy ending. Top work Db There is so much talent on this site

I just happened to notice the gold nichicon caps....Did you recap this as well?

 

[Dbzerk]

Good eye! Yup while I had her opened up did a full cap change over, thank you to Caution for the caps list.
Also reflowed all the solder joints.
Desoldered all switches and pots for full tear down clean and deoxit.

I'll post up a new thread of the tear down soon. Hope to get her back together today.

 

[Fatdog]

I never cease to be amazed at the talent and ingenuity of some members here. Top work Db!

 

[Dbzerk]

I never cease to be amazed at the talent and ingenuity of some members here. Top work Db!

Thanks mate! But the real thanks goes to this site!

We have some truly valuable members on here that I have learned a lot from.
It's a great community that keeps on giving.